By virtue of excellent magnetic properties, Nd—Fe—B permanent magnets find an ever increasing range of application. The recent challenge to the environmental problem has expanded the application range of these magnets from household electric appliances to industrial equipment, electric automobiles and wind power generators. It is required to further improve the performance of Nd—Fe—B magnets.
Indexes for the performance of magnets include remanence (or residual magnetic flux density) and coercive force. An increase in the remanence of Nd—Fe—B sintered magnets can be achieved by increasing the volume factor of Nd2Fe14B compound and improving the crystal orientation. To this end, a number of modifications have been made. For increasing coercive force, there are known different approaches including grain refinement, the use of alloy compositions with greater Nd contents, and the addition of coercivity enhancing elements such as Al and Ga. The currently most common approach is to use alloy compositions having Dy or Tb substituted for part of Nd.
It is believed that the coercivity creating mechanism of Nd—Fe—B magnets is the nucleation type wherein nucleation of reverse magnetic domains at grain boundaries governs a coercive force. In general, a disorder of crystalline structure occurs at the grain boundary or interface. If a disorder of crystalline structure extends several nanometers in a depth direction near the interface of grains of Nd2Fe14B compound which is the primary phase of the magnet, then it incurs a lowering of magnetocrystalline anisotropy and facilitates formation of reverse magnetic domains, reducing a coercive force (see Non-Patent Document 1). Substituting Dy or Tb for some Nd in the Nd2Fe14B compound increases the anisotropic magnetic field of the compound phase so that the coercive force is increased. When Dy or Tb is added in an ordinary way, however, a loss of remanence is unavoidable because Dy or Tb substitution occurs not only near the interface of the primary phase, but even in the interior of the primary phase. Another problem arises in that amounts of expensive Tb and Dy must be used.
Besides, a number of attempts have been made for increasing the coercive force of Nd—Fe—B magnets. One exemplary attempt is a two-alloy method of preparing an Nd—Fe—B magnet by mixing two powdered alloys of different composition and sintering the mixture. Specifically, a powder of alloy A consisting of R2Fe14B primary phase wherein R is mainly Nd and Pr, and a powder of alloy B containing various additive elements including Dy, Tb, Ho, Er, Al, Ti, V, and Mo, typically Dy and Tb are mixed together. This is followed by fine pulverization, molding in a magnetic field, sintering, and aging treatment whereby the Nd—Fe—B magnet is prepared. The sintered magnet thus obtained produces a high coercive force while minimizing a decline of remanence because Dy and Tb are absent at the center of R2Fe14B compound primary phase grains and instead, the additive elements like Dy and Tb are localized near grain boundaries (see Patent Documents 1 and 2). In this method, however, Dy and Tb diffuse into the interior of primary phase grains during the sintering so that the layer where Dy and Tb are localized near grain boundaries has a thickness equal to or more than about 1 micrometer, which is substantially greater than the depth where nucleation of reverse magnetic domains occurs. The results are still not fully satisfactory.
Recently, there have been developed several processes of diffusing certain elements from the surface to the interior of a R—Fe—B sintered body for improving magnet properties. In one exemplary process, a rare earth metal such as Yb, Dy, Pr or Tb, or Al or Ta is deposited on the surface of Nd—Fe—B magnet using an evaporation or sputtering technique, followed by heat treatment, as described in Patent Documents 3 to 5 and Non-Patent Documents 2 and 3. Another exemplary process involves applying a powder of rare earth inorganic compound such as fluoride or oxide onto the surface of a sintered body and heat treatment as described in Patent Document 6. With these processes, the elements (e.g., Dy and Tb) disposed on the sintered body surface pass through grain boundaries in the sintered body structure and diffuse into the interior of the sintered body during the heat treatment. As a consequence, Dy and Tb can be enriched in a very high concentration at grain boundaries or near grain boundaries within sintered body primary phase grains. As compared with the two-alloy method described previously, these processes produce an ideal morphology. Since the magnet properties reflect the morphology, a minimized decline of remanence and an increase of coercive force are accomplished. However, the processes utilizing evaporation or sputtering have many problems associated with units and steps when practiced on a mass scale and suffer from poor productivity.
Besides the foregoing methods, Patent Document 6 discloses a method comprising coating a surface of a sintered body with a powdered rare earth inorganic compound such as fluoride or oxide and heat treatment, and Patent Document 8 discloses a method comprising mixing an Al, Cu or Zn powder with a fluoride, coating a magnet with the mixture, and heat treatment. These methods are characterized by a very simple coating step and a high productivity. Specifically, since the coating step is carried out by dispersing a non-metallic inorganic compound powder in water, immersing a magnet in the dispersion and drying, the step is simple as compared with sputtering and evaporation. Even when a heat treatment furnace is packed with a large number of magnet pieces, the magnet pieces are not fused together during heat treatment. This leads to a high productivity. However, since Dy or Tb diffuses through substitution reaction between the powder and the magnet component, it is difficult to introduce a substantial amount of Dy or Tb into the magnet.
Further Patent Document 7 discloses coating of a magnet body with a mixture of an oxide or fluoride of Dy or Tb and calcium or calcium hydride powder, followed by heat treatment. During the heat treatment, once Dy or Tb is reduced utilizing calcium reducing reaction, Dy or Tb is diffused. The method is advantageous for introducing a substantial amount of Dy or Tb into the magnet, but less productive because the calcium or calcium hydride powder needs careful handling.
Patent Documents 9 to 13 disclose coating of the sintered body surface with a metal alloy instead of a rare earth inorganic compound powder such as fluoride or oxide, followed by heat treatment. The method of coating with only metal alloy has the drawback that it is difficult to coat the metal alloy onto the magnet surface in a large and uniform coating weight. In Patent Documents 14 and 15, a metal powder containing Dy and/or Tb is diffused into the mother alloy. The oxygen concentration of the mother alloy is restricted below 0.5% by weight, and the rare earth-containing metal powder is closely contacted with the mother alloy by a barrel painting technique of oscillating impact media within a barrel for agitation. Diffusion takes place under these conditions. However, this method requires many steps as compared with the method of coating a mother alloy magnet with a dispersion of a powder mixture of an intermetallic compound and a rare earth oxide in a solvent. The method is time consuming and is not industrially useful.